An engine comprises: a cylinder block having a bore; a piston fitted into the bore to be capable of reciprocating therein so as to form a combustion chamber therein; a cylinder head having a valve bore closing the combustion chamber and communicating with the combustion chamber; and a valve opening and closing the valve bore. In order to upgrade fuel consumption or mileage, it is preferable to enhance the combustion chamber in the heat-insulating property. In particular, in vehicles intending to upgrade the mileage, such as hybrid vehicles or vehicles provided with an idling stop function, driving the engine is sometimes brought to a halt temporarily during travelling the vehicles, or during bringing the vehicles to a halt temporarily. Under the circumstances, since the temperatures in the combustion chamber of the engine tend to drop, there are limitations in upgrading the mileage of the engine.
Patent Literature No. 1 discloses a piston in which a low thermal-conductive member is coated on a top face of the piston body. In the literature, the low thermal-conductive member is formed of a metallic material (such as titanium) whose thermal conductivity is lower than that of an aluminum material forming the piston body, and air films for heat insulation are further formed between the low-thermal conductive member and the piston body's top face. Patent Literature Nos. 2 and 3 disclose an engine, in which a heat-insulative material is formed on the top face of a piston by thermal spraying ceramic, respectively. Patent Literature No. 4 discloses a painted metallic plate made by forming a heat-insulative painted layer, which has hollow particles with an average particle diameter of from 5 to 27 μm therein, onto a surface of the metallic plate. Patent Literature No. 5 discloses a technique for forming an anode-oxidized coated film, whose porosity is 20% or more, onto an inner face of an engine's combustion chamber. Patent Literature No. 6 has mentions on a heat-insulative film in which a resinous material and hollow particles with an average particle diameter of from 5 to 15 nanometers are blended.
However, in Patent Literature No. 1, aluminum used for the piston has a specific gravity of 2.7, a thermal conductivity of 130 W/mK, and a thermal-expansion coefficient of 23×10−6/° C.; whereas titanium used for the heat-insulative material has a specific gravity of 4.5, a thermal conductivity of 17 W/mK, and a thermal-expansion coefficient of 8.4×10−6/° C. In order to demonstrate sufficient heat insulation with the heat-insulative material comprising titanium, it is necessary to make the heat-insulative material have a thickness on an order of millimeter. On the contrary, titanium is heavier than aluminum. Hence, when titanium is used for the heat-insulative material, it results in a weight increment for the piston reciprocating at high speeds, thereby hindering upgrading the mileage. Moreover, due to the weight and thickness of the heat-insulative material, and due to the differences between the thermal-expansion coefficients of the heat-insulative material and piston, it is not possible to maintain the strength in a joined face between the heat-insulative material and the piston.
In Patent Literature Nos. 2 and 3, since heat-insulative materials comprising thermal-sprayed ceramic are used, the face subjected to the thermal spraying has been more roughened after the thermal-spraying treatment than before the thermal-spraying treatment. When a heat-insulative material comprising thermal-sprayed ceramic is formed on the top face of a piston, protrusions with fine surface roughness turn into heat spots making the factor of ignition, so that they are likely to become the cause of knocking in engine. Moreover, since a heat-insulative material comprising thermal-sprayed ceramic is hard, it is difficult to do post-processing to the heat-insulative material.
When the painted metallic plate disclosed in Patent Literature No. 4 is used for internal combustion engine, it has limitations in the blending amount of the hollow particles within the painted film formed on the metallic plate's surface.
Although Patent Literature No. 5 contains mentions on a heat-insulative film made by an anode-oxidation treatment, the face subjected to the treatment has been more roughened after the treatment than before the treatment. Accordingly, when the top face of a piston is subjected to the anode-oxidation treatment, protrusions with fine surface roughness turn into heat spots making the factor of ignition, so that they are likely to become the cause of knocking in engine. The heat-insulative film disclosed in Patent Literature No. 6 comprises hollow particles and a resinous material, but has limitations in the heat-insulating property and strength of the coated film in order to maintain the film's formability. Moreover, the heat-insulative film's heat resistance is insufficient.
Hence, the present inventors have been seeking earnestly for ways in order for forming heat-insulation coating films provided with higher heat-insulating property and higher superficial flatness/smoothness. In recent years, it has been required for heat-insulation coating films to deal with engines with a higher compression ratio. Under such circumstances, it has been needed more and more to enhance heat-insulative films in the superficial flatness/smoothness as well as in the heat-insulating property.